The present invention generally relates to an electrochemical cell construction. More particularly, the present invention relates to the construction of an electrochemical cell that provides support for a low-profile collector assembly.
FIG. 1 shows the construction of a conventional C-sized alkaline cell 10. As shown, cell 10 includes a cylindrically-shaped can 12 having an open end and a closed end. Can 12 is preferably formed of an electrically-conductive material such that an outer cover (not shown) welded to a bottom surface 14 at the closed end of can 12, serves as an electrical contact terminal for the cell.
Cell 10 further typically includes a first electrode material 15, which may serve as the positive electrode (also known as a cathode). The first electrode material 15 may be preformed and inserted into can 12 or, more preferably, may be molded in place so as to contact the inner surfaces of the can 12. After the first electrode 15 has been provided in can 12, a separator 17 is inserted into the space defined by first electrode 15. Once separator 17 is in place within the cavity defined by first electrode 15, an electrolyte is dispensed into the space defined by separator 17 along with a mixture 20 of an electrolyte and a second electrode material, which may be the negative electrode (also known as the anode). The electrolyte/second electrode mixture 20 preferably includes a gelling agent.
After the first electrode 15, separator 17, the electrolyte, and mixture 20 have been formed inside can 12, a preassembled collector assembly 25 is inserted into the open end of can 12. Can 12 is typically slightly tapered to have a larger diameter at its open end. This taper serves to support the collector assembly in a desired orientation prior to securing it in place. After collector assembly 25 has been inserted, an outer cover 45 is placed over collector assembly 25. Collector assembly 25 and outer cover 45 are secured in place by radially squeezing and crimping an upstanding wall of collector assembly 25 and outer cover 45 within the end edge 13 of can 12. As described further below, the primary function served by collector assembly 25 is to provide for a second external electrical contact for the electrochemical cell. Additionally, collector assembly 25 must seal the open end of can 12 to prevent the electrochemical materials therein from leaking from the cell.
The collector assembly 25 shown in FIG. 1 includes a seal 30, a collector nail 40, an inner cover 44, a washer 50, and a plurality of spurs 52. Seal 30 is shown as including a central hub 32 having a hole through which collector nail 40 is inserted. Seal 30 further includes a V-shaped portion 34 that may contact an upper surface 16 of first electrode 15 and to provide a spring-like force radially outward.
Seal 30 also includes a peripheral upstanding wall 36 that extends upward along the periphery of seal 30 in an annular fashion. Peripheral upstanding wall 36 not only serves as a seal between the interface of collector assembly 25 and can 12, but also serves as an electrical insulator for preventing an electrical short from occurring between the positive can and negative contact terminal of the cell.
Inner cover 44, which is formed of a rigid metal, is provided to increase the rigidity and to support the radial compression of collector assembly 25 thereby improving the sealing effectiveness. As shown in FIG. 1, inner cover 44 is configured to contact central hub portion 32 and peripheral upstanding wall 36. By configuring collector assembly 25 in this fashion, inner cover 44 serves to enable compression of central hub portion 32 by collector nail 40 while also supporting compression of peripheral upstanding wall 36 by the inner surface of can 12.
Outer cover 45 is typically made of a nickel-plated steel and is configured to extend from a region defined by the annular peripheral upstanding wall 36 of seal 30 and to be in electrical contact with a head portion 42 of collector nail 40. Typically, outer cover 45 is welded to head portion 42 of collector nail 40 to prevent any loss of contact. As shown in FIG. 1, when collector assembly 25 is inserted into the open end of can 12, collector nail 40 penetrates deeply within the electrolyte/second electrode mixture 20 to establish sufficient electrical contact therewith.
In the example shown in FIG. 1, outer cover 45 includes an upstanding wall 47 that extends vertically upward along the circumference of outer cover 45. By forming peripheral upstanding wall 36 of seal 30 of a length greater than that of upstanding wall 47, a portion of peripheral upstanding wall 36 may be folded over upstanding wall 47 during the crimping process so as to prevent any portion of the upper edge 13 of can 12 from coming into contact with outer cover 45.
Seal 30 is preferably formed of nylon. In the configuration shown in FIG. 1, a pressure relief mechanism is provided for enabling the relief of internal pressure when such pressure becomes excessive. Further, inner cover 44 and outer cover 45 are typically provided with apertures (not shown) that allow the hydrogen gas to escape to the exterior of cell 10. The mechanism shown includes an annular metal washer 50 and a plurality of spurs 52 that are provided between seal 30 and inner cover 44. The plurality of spurs 52 each include a pointed end 53 that is pressed against a thin intermediate portion 38 of seal 30. Spurs 52 are biased against the lower inner surface of inner cover 44 such that when the internal pressure of cell 10 increases and seal 30 consequently becomes deformed by pressing upward toward inner cover 44, the pointed ends 53 of spurs 52 penetrate through the thin intermediate portion 38 of seal 30 thereby rupturing seal 30 and allowing the escape of the internally-generated gas.
Although the above-described collector assembly 25 performs all the above-noted desirable functions satisfactorily, as apparent from its cross-sectional profile, this particular collector assembly occupies a significant amount of space within the interior of the cell 10. Because the interior dimensions of the electrochemical cell are generally fixed, the greater the space occupied by the collector assembly, the less space that there is available within the cell for the electrochemical materials. Consequently, a reduction in the amount of electrochemical materials that may be provided within the cell results in a shorter service life for the cell. Collector assemblies have subsequently been designed that have a lower profile and thus occupy less space within the electrochemical cell.
To reduce the profile of the collector assembly, the V-shaped portion 34 is typically eliminated leaving a flat bottom surface on the collector assembly that opposes the upper surface 16 of first electrode 15. Also, the area in which the collector assembly contacts the inner surface of the can is also reduced. These changes are not without problems. By reducing the can/collector assembly contact area, the friction fit of the collector assembly in the can becomes less secure thereby making it more likely that the collector assembly will move during crimping. Thus, some additional structure must be provided to support the collector assembly during crimping. If the collector assembly is left without additional support during crimping, a low-profile collector assembly will most likely be cocked with respect to the can when it is crimped in place. When the collector assembly is cocked, it will not create an adequate seal at the open end of the can and will be very likely to leak.
One technique used to support the collector assembly is to bead the can about its circumference in an area just above first electrode 15, as shown in FIG. 2. By providing a bead 102, a collector assembly, such as the low-profile collector assembly 125 shown in FIG. 2, may be inserted in the open end of can 12 and supported on bead 102 while end 103 of can 12 is crimped down on collector assembly 125. Further, by providing a sturdy support for the collector assembly, a lower profile crimp may be utilized that applies a downward axial force against the collector assembly with edge 103 of can 12 so that the collector assembly is pinched between bead 102 and edge 103. As compared with the crimp profile shown in FIG. 1, the low-profile crimp shown in FIG. 2 allows for more space in the interior of the cell.
Although the beading technique works satisfactorily when the first electrode 15 is preformed as a plurality of annular rings 15a and 15b that are stacked within can 12 prior to beading of can 12, the technique does require that beading take place after first electrode 15 has been inserted. Further, when first electrode 15 is molded in place within can 12, the beading must also take place after first electrode 15 has been inserted, otherwise molding the upper surface of first electrode 15 in the vicinity of bead 102 would be difficult. Furthermore, regardless of the method used to deposit first electrode 15 in can 12, bead 102 takes up additional space within the interior of the electrochemical cell.
As a solution to the foregoing problems, it has been proposed to support the collector assembly directly on the upper surface 16 of first electrode 15. However, if the first electrode material 15 is deposited in can 12 as a plurality of stacked preformed annular rings 15a and 15b, the stacked rings do not provide uniform electrode surface height from cell to cell to consistently support the collector assembly. If the first electrode material 15 is molded in place, flashing 18 (FIG. 3) is typically formed along the side interior walls of can 12 above the upper surface (16) of the first electrode (15). The formation of this flashing is problematic in that it will either come between the peripheral edge of the collector assembly and the inner surface of the can or get folded over when the collector assembly is inserted in a manner similar to that shown in FIG. 3. If the collector assembly 125 rests on top of flashing 18, collector assembly 125 will be cocked with respect to can 12 thereby increasing the likelihood that the cell closing will be ineffective by resulting in a leaking cell. If flashing 18 comes between a peripheral edge 116 of collector assembly 125 and the side wall of can 12, collector assembly 125 may also be cocked. In either event, the existence of the flashing is likely to cause leakage.
Because the removal of flashing is not without cost due to the difficulty removing all MnO.sub.2 residue on the can wall, there exists a need for a different configuration to support a collector assembly in an electrochemical cell without requiring any significant amount of space within the interior cell to be occupied by structure other than the active electrochemical materials of the cell.